O-specific polysaccharide of Shigella sonnei (O-SP of S. sonnei)

The O-specific polysaccharide of Shigella sonnei (O-SP of S. sonnei) is a high-molecular-weight carbohydrate structure forming the outermost part of the lipopolysaccharide (LPS) on the bacterium’s surface, and can also be found as a capsule not linked to LPS in S. sonnei. It is composed principally of repeating units of two rare sugars: 2-acetamido-2-deoxy-L-altruronic acid and 2-acetamido-2-deoxy-L-fucose, which are not found in other Shigella serogroups. This antigenic polysaccharide is a dominant virulence factor and the primary target for protective humoral immunity in shigellosis. Covalent conjugation of O-SP to carrier proteins greatly enhances its immunogenicity, forming the basis for candidate and investigational vaccines against S. sonnei infection, especially for young children who otherwise respond poorly to pure polysaccharide antigens. The O-specific polysaccharide plays a dual role in protecting the bacterium from host immune mechanisms (such as complement-mediated lysis), and, depending on its presence and configuration, can modulate invasiveness and inflammation. Diagnostic and vaccine designs targeting this molecule exploit its immunodominance and serotype specificity.

Other names

O-antigen polysaccharide of Shigella sonneiS. sonnei O-SPS. sonnei O-antigenOAg capsule of Shigella sonnei

Induction of protective antibody responses (when used as vaccine antigen). Inhibition of immune system recognition (native bacterial role). Resistance to complement-mediated killing (capsule-mediated). Modulation of bacterial invasiveness and systemic dissemination.